The invention relates to dental compositions on the basis of polyether derivatives, to their preparation and to their use as impression materials.
The preparation of polyether derivatives and their use in dental materials has long been known. Thus, for example, DE-C-1 745 810 describes the preparation of mouldings on the basis of aziridino polyethers.
The use of aziridino polyethers in polyether impression compositions is described in patent specifications DE-C-3 246 654, EP-A-0 421 371 and EP-A-0 110 429. EP-A-0 460 478 describes light-curing impression materials on the basis of polyether urethane acrylates.
To take impressions of the concrete conditions in the patient""s mouth with the help of suitable impression compositions is the prerequisite for the preparation of precisely fitting denture sets, crowns and bridges, inlays and onlays.
Of the known impression compositions, the compositions based on polyether derivatives are characterized by their hydrophilic character, which makes possible a very high precision of the impressions.
However, a disadvantage of these compositions is that they cannot be demoulded too easily. In other words, the demouldability of the copy when the impression is taken and the demouldability of the gypsum model after forming the cast of the impression are not satisfactory.
The object of the present invention is to provide dental compositions on the basis of polyether derivatives which do not display the described disadvantages but are easily demouldable.
This object is achieved by dental compositions on the basis of polyether derivatives which are characterized in that their level of cyclic oligomeric polyethers is smaller than 5.0, preferably smaller than 0.9 wt.-%.
Surprisingly, it was found within the framework of the present invention that the cyclic polyether oligomers present in the polyether compositions are responsible for the compositions displaying a poor demouldability of the copy when the impression is taken and a poor demouldability of the gypsum model after forming the cast of the impression.
The basis for the polyether derivatives used in dental materials is polyether polyols which can be prepared by different polymerization processes and generally have molar masses in the range from 500 to 10,000 g/mol.
It is already known (G. Pruckmayr et al., ACS Symp. Ser. 172, (1981), pp. 197 to 203) that, during the preparation of polyether glycols by homopolymerization of tetrahydrofuran or ethylene oxide or by copolymerization of tetrahydrofuran with ethylene oxide under the catalytic action of strong acids, cyclic oligomers form as well as linear polyethylene glycols and that the level of such cyclic oligomers can be up to 20 wt.-% depending on the reaction conditions (DE-A-3 514 547) and is usually below 10 wt.-% in the case of industrial polyether polyols.
These cyclic oligomers have different ring sizes and/or different ratios of the incorporated monomer units depending on the comonomer composition and the reaction conditions.
Processes which relate to the removal of the cyclic oligomers are also described in the state of the art. Thus EP-A-0 153 794 describes the removal of the cyclic oligomers through extraction by means of heptane.
Extraction processes for the removal of the cyclic oligomers are also described in DE-A-3 514 547, DE-A-3 607 946 and DE-A-3 730 888.
A three-phase extraction with upstream vacuum distillation in a short-path evaporator is proposed according to EP-A-0 305 853.
Low-odour, higher-molecular-weight polyether polyols which are purified by adding water at temperatures of 110 to 150xc2x0 C. and under reduced pressure are described in DE-A-195 30 388. This purification process leads to a reduction in the level of odour-intensive compounds and makes the polyethers purified in this way suitable for the preparation of low-emission polymers, cosmetics and pharmaceutical products building on polyether polyols.
In the case of dental compositions the removal of the cyclic oligomers has not been considered thus far, since these dental compositions display excellent properties and thus there was no need to remove the cyclic oligomers. Surprisingly, as already mentioned above, it was found within the framework of the invention that the difficulty in demoulding the polyether compositions is to be attributed to the presence of the cyclic oligomers. Through their removal, dental compositions on polyether basis are obtained which possess an impression precision which remains good, but are also easily demouldable at the same time.
With a pre-set incorporation ratio of the monomers, the overall level of cyclic oligomeric polyethers, the incidence of the individual types relative to one another and thus the molar mass distribution of the cyclic oligomers can be influenced through the reaction temperature and the realized concentration pattern of the monomers via the reaction time.
The analytical determination of the level of cyclic oligomeric polyethers and the incidence of the individual oligomer types can be realized by means of gas chromatography with a FID detector or in GC-MS coupling.
The removal of the cyclic oligomeric polyethers can take place both at the process stage of the polyether polyols and after their functionalization with aziridino groups, double-bond-containing groups and epoxide groups, methods based on distillation and extraction or membrane separation being usable.
The separation methods based on distillation are associated with the application of high temperatures and thus the dangers of thermal damage. Thus, the polyether polyols which are cleaned up by means of distillation can have an odour which is unpleasant in most cases.
The distillation-based purification of the already functionalized polyether polyols is difficult to carry out on account of the danger of premature polymerization. Extraction with hydrocarbons having 4 to 12 C atoms is suitable in principle for the extraction-based removal of the cyclic oligomeric polyethers. The extraction can be carried out continuously or batchwise according to the known liquid/liquid separation methods (see Ullmann""s Encyclopedia of Industrial Chemistry, 5th Edition, Volume B3: Unit Operations). The extraction-based removal of the cyclic oligomeric polyethers by means of hydrocarbons is possible both at the stage of the polyether polyols and following the functionalization. It has proved advantageous, during the preparation of polyether derivatives for use in dental materials, to combine the purification stage of the removal of the cyclic material through extraction with the purification stages after the functionalization.
The polyether polyols used for the functionalization are preferably prepared by copolymerization of tetrahydrofuran and ethylene oxide in the molar ratio 10:1 to 1:1 and preferably 5:1 to 3:1 in the presence of strong acid, such as for example boron fluoride etherates.
It is also possible to use, for the functionalization, polyether polyols which, in addition to tetrahydrofuran units, also contain ethylene oxide units and or propylene oxide units.
The polyether polyols possess at least 2 hydroxyl groups, but can also contain up to 20 hydroxyl groups per molecule.
The molar masses (Mn) of the polyether polyols used for the functionalization lie in the range from 500 to 20,000 and preferably in the range from 2,000 to 10,000 g/mol. The functionalization with aziridino groups can take place for example according to the method described in DE-C-1 745 810.
A functionalization with epoxide groups, such as for example 3,4-epoxycyclohexyl groups, is possible according to the teaching of DE-A-195 34 668. The functionalization with (meth)acrylate groups can take place for example in accordance with DE-A-4 406 858, Example 1. The known methods of conversion of primary alcohols can be used for the functionalization with allyl groups, vinyl groups, vinyl ether groups and maleate groups.
Dental compositions which cure through polymer-forming reactions can be manufactured from the functionalized polyether derivatives. Preferred polymer-forming reactions are free radical and cationic polymerization and also hydro-silylation.
The use of the functionalized polyether derivatives which are largely freed of cyclic oligomeric polyethers can take place in very different dental compositions used in dental medicine or dental engineering. Preferred fields of use are single-phase and two-phase dental medicine impression taking and occlusion recording.
The invention is explained further with the aid of the following examples.